Somatic cell (lymphocyte-myeloma) hybrids from fusions of long-lived cell lines with antibody-forming cells from mice immunized with Type III pneumococcal polysaccharide (SSS-III) were used to investigate several basic issues associated with the antibody responses to SSS-III. The results obtained showed that the plaque-forming cell (PFC) repertoire expressed after immunization with an optimally immunogenic dose of SSS-III was reiterated in the distribution of isotypes found among hybrids making antibody specific for this antigen. Using the monoclonal products of hybridomas as probes to examine the diversity of the V-regions of antibody specific for SSS-III, it was found that the CC4.6 cross reactive idiotype is common and expressed on several classes of immunoglobulin with specificity for SSS-III.